# MCQs on Electrical Machines

##### Page 22 of 114. Go to page 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114
01․ In speed control of DC shunt motor by varying field flux, what may be the value of field regulator?
0.2 ohm
2 ohm.
10 ohm
100 ohm

We know the flux is produced by field current. In shunt motor, field regulator is connected in series with the shunt field winding. The connection of field regulator reduces the field current and hence the flux φ is also reduced. The reduction of flux will result in an increase in the speed. If we want to reduce shunt field current then the resistance of field regulator would be a higher value.

02․ Which of the following is done in speed control of DC motor by field flux control?
Inserting resistance to armature circuit and field circuit.
Inserting an additional resistance in the armature circuit.
Inserting an additional resistance in the field circuit.
Any of above.

The flux is produced by field current. Inserting a resistance reduces field current and hence the flux is also reduced. The reduction in flux will result in an increase in speed. Because of saturation of iron, the flux cannot be increased beyond its normal value. So the motor runs at a speed higher than normal speed.

03․ In field flux method speed control of DC Motor is
constant power drive above base speed.
constant hp drive below the base speed.
constant torque drive above the base speed.
constant torque drive below the base speed.

Since the flux cannot be increased beyond its normal value because of saturation of iron, so speed control by flux is limited. So it is applicable only over a limited range, because if the field is weakened too much there is a loss of stability. Hence field flux method is generally used constant power drive above the base speed. Speed increases because speed is inversely proportional to flux.

04․ DC Shunt Motor has disadvantage(s) of
constant speed.
less expensive.
not suitable for rapidly changing loads.
none of above.

In DC shunt motor, speed variation depends not only upon the controlling resistance but on the load current also. This double dependance makes it impossible to keep the speed sensibly constant on rapidly changing load.

05․ Ward-Leonard system for controlling the speed of a DC motor is used where
The accuracy in speed control is required.
The speed in both the direction is required.
Very fine speed control over the whole range from zero to normal speed is required.
All of above.

Ward-Leonard system is used for speed control of DC motor where speed variation in both direction, accuracy in speed control or very fine speed control over wide range are required and uniform acceleration can be obtained. Such a system is required in steel rolling mills, paper machine, elevators, cranes, mine hoists etc.

06․ The speed of a cumulatively compound DC motor can be controlled by varying
Armature resistance control.
Shunt field resistance control.
Armature voltage control.
Any of above.

The speed control of a cumulatively compound motor can obtained by varying any of one following - 1. Armature voltage control 2. Armature resistance control 3. Field resistance / field flux control.

07․ In Ward-Leonard method of speed control, the direction of rotation of motor is reversed usually reversing the connection of the
motor armature terminal.
generator armature terminal.
generator field terminal.
motor field terminal.

The direction of rotation of motor armature can be reversed by reversing the direction of exciting current of the generator. This is done with help of reversing switch.

08․ The function of field regulator in a DC compound motor is to
control field current.
limit the armature current.
vary the armature resistance.
none of above.

In DC compound motor, the field regulators are inserted in series with shunt field to control its field current.

09․ Which of the following represents the pitch factor?
Cos α
Cos α²
Cos (α/2)
sin (α/2)

Pitch factor is defined as the ratio of emf induced in the short pitch winding or coil to the emf induced in the full pitch winding or coil. Due to short pitched coils there is a reduction in voltage which is known by pitch factor Kp.

10․ A 3-phase, 2 pole, 12 slot synchronous machine is operating at synchronous speed. Find the slot angle?
20°
30°
18°
60°

<<<2021222324>>>